Insulin from beta cells will lower blood glucose levels
After eating a meal containing carbohydrates, such as sugary or starchy foods, blood glucose levels will start to rise. These rising levels of blood sugar are detected by the beta cells in the pancreatic islets. The same beta cells respond by secreting insulin. Insulin is released directly into the blood where it facilitates the conversion of soluble glucose into insoluble glycogen which can then be stored in the liver and muscles. When glucose is converted to glycogen it is no longer in the blood,
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but being stored in the muscles and liver in insoluble form. Therefore blood sugar levels will drop down, back towards a normal homeostatic level.
In addition to this mechanism, insulin also facilitates the transfer of glucose across cell membranes, from the tissue fluids and blood, into the cells of the body. In the absence of insulin, most cell membranes are impervious to the passage of glucose. This is because glucose is a water soluble molecule while cell membranes are lipid based. When an insulin molecule arrives at a cell membrane it combines with a specific insulin receptor protein in the cell membranes. The combination of the insulin and the insulin receptor trigger changes within the cell which results in the opening of glucose channels or 'gates' through which glucose may pass. This means more glucose, under the influence of insulin, passes from the tissue fluids of the body into the cells. Once in the cells, glucose may be metabolised in the mitochondria to produce energy. Clearly, if the glucose is in the cells it is no longer in the blood, so again blood levels are lowered. The combination of the glucose to glycogen mechanism and the transport of glucose into cells means that insulin causes the level of blood glucose to drop back down towards a normal homeostatic level. Insulin is actually a small protein made up of 51 amino acid units. This is why insulin used therapeutically must be injected if it is required; if it were swallowed the protein digesting enzymes in the stomach would simply digest it making it ineffective.